A multi-channel detector has a number of detector elements or pixels arranged in rows and columns which form the individual channels of the radiation detector. With a quanta-counting radiation detector a charge pulse is generated for each detected radiation quantum in the respective detector element. The charge pulse is shaped by the detector electronics and the resulting electrical signal is compared in one or more comparators with a threshold value. If the threshold value is exceeded, a counter is incremented. The radiation quanta counted within an integration time produce the measurement signal for the respective comparator channel.
Since the size of the charge pulse depends on the energy of the incident radiation quantum a spectral selection of the counted radiation quanta can be reached via a setting of the electrical threshold level or of the threshold value of the comparator. Only the radiation quanta which as a result of their energy generate an electrical signal which exceeds the threshold value comparator are counted.
Because of variations in the detector elements or pixels and the electronics, the electrical threshold level of the comparator corresponding to a specific energy of the radiation quanta is different for each channel. Thus in the calibration of this type of radiation detector the electronic threshold levels must be set separately for each comparator and each channel so that they correspond to a predetermined energy of the incident radiation quanta. During energy calibration it is determined, for different threshold values of the respective comparator, which energy threshold corresponds to this threshold value. For energy calibration of quanta-counting x-ray detectors radioactive preparations, synchrotron light sources or K fluorescence radiators are used, which emit defined spectral lines or quanta. As a result of the energy calibration each threshold value of the comparator is assigned an energy threshold.
Only a limited accuracy is achieved with the previously known methods for energy calibration however, so that the setting of the threshold levels of the comparators of the radiation detector also only makes possible a restricted accuracy. This results in a dispersion of the actual energetic threshold level over the channels of the detectors, also referred to below as the threshold dispersion. This threshold dispersion has the effect that empty images of the detector measured at different energetic distributions of the incident radiation, also known by the terms “Airscan” or “Flat field image”, cannot be converted into one another by a simple scaling of the count rates. The effort for further processing of the measurement data of the detector is significantly increased by this.